High electrical and thermal conductive polymer composites attract considerable interest in the conductive composite industry due to their wide applications in electronic industries. Examples of high electrical and thermal conductive polymer composites useful in electronic industries include, but are not limited to, die attach materials for thermal management, liquid crystal displays (LCD), surface-mounted assemblies of packaged components on printed wiring board (PWB's), radio frequency identification (RFID) tags, organic light emitting diodes (OLEDs), thin-film photovoltaics (PV), membrane switches, touch panels, printed batteries and sensors, greeting cards, and toys. Typically, these conductive composites contain a polymer matrix filled with conductive fillers. The composite, with a composition beyond a critical volume fraction, transforms from an insulator to a conductor while the conductive fillers form a continuous network. This transformation is known as the connective percolation theory. A disadvantage of using these conventional conductive polymer composites is their relative high resistivity. As a result, many efforts have been dedicated to enhancing the electrical and thermal conductivity of conductive polymer composites, such as through filler selection and modifications.
Conductive silver pastes and inks are widely used in the field of printed electronics and rapid growth in the field continues to promote the consumption of conductive materials. In the past, there have been attempts to lower the percolation threshold and to enhance the conductivity of conductive composites by using long-aspect ratio nanomaterials, such as single-wall nanotubes (SWNT's) and silver nanowires. However, these materials are extremely difficult to handle and incorporate into scalable fabrication, and can only be acquired at a high cost. Consequently, nanomaterials are difficult to commercialize and impractical to use in lowering the percolation threshold and enhancing the conductivity of conductive composites.
As discussed by Peter Lu et al. in Colloidal particles: crystals, glasses, and gels (hereinafter, “Colloidal particles”) and in Gelation of particles with short-range attraction (hereinafter, “Gelation of particles”) [Lu, Peter J. et al. Colloidal particles: Crystals, glasses, and gels. Annual Review of Condensed Matter Physics, Vol 4, 4:217-233 (2003)] [Lu, Peter J. et al. Gelation of particles with short-range attraction. Nature 453, 499-503 (2008)], it is well known that colloidal suspension, including small solid particles with sizes ranging between 10 nanometers (nm) and several microns suspended in a fluid and driven by thermal energy, form fractal cluster-like networks during a gelation process. In these fractal clusters, a small number of particles occupy a large volume of space. For example, FIG. 1 shows a typical transmission electron microscope (TEM) image of a cluster jammed from a gold colloidal nanoparticle where the size of the gold nanoparticles is approximately 10 nm, and its cluster size is approximately 1 micrometer (μm).
Furthermore, P. J. Lu et al. in Gelation of particles discuss that colloidal particle gelation has been widely investigated in attractive colloidal systems. FIGS. 2a and 2b show 3D confocal microscope images of a standard colloidal suspension consisting of microspheres with a particle size of approximately 1 μm and a particle volume fraction (v %) of 4.5, indicating the formation of a 3D spanning particle network. Additionally, as shown in the inset of FIGS. 2a and 2b are the 2D confocal microscope images of the colloidal particles before and after gelation, which clearly demonstrate a fractal network. The fractal cluster size ranges between approximately 20 μm to 50 μm. Although the fractal cluster size is similar to the size of a jamming percolating network, using a jamming gelation technique to control percolation and decrease the filler content is not considered in prior art.
Furthermore, Fortini et al. investigate in Clustering and gelation of hard spheres induced by the Pickering effect [A. Fortini. Clustering and gelation of hard spheres induced by the Pickering effect. Phys. Rev. E 85, 040401(R) (2012)] the gelation of colloidal particles with a volume loading of 10 v % under capillary attraction by Brownian dynamics simulation, as shown in FIGS. 3a-3d. Fortini et al. also reveal the formation of a fractal clustering network by capillary bridging of the colloidal particles. However, Fortini et al. fail to control percolation, decrease the filler content, or use a jamming gelation technique.